OK, let's work through this together. We're starting with 775 and we're going to multiply it by 21, which means we add 775 to itself 21 times.
Step 1: 0 + 775 = 775
Step 2: 775 + 775 = 1550
Step 3: 1550 + 775 = 2325
Step 4: 2325 + 775 = 3100
Step 5: 3100 + 775 = 3875
Step 6: 3875 + 775 = 4650
Step 7: 4650 + 775 = 5425
Step 8: 5425 + 775 = 6200
Step 9: 6200 + 775 = 6975
Step 10: 6975 + 775 = 7750
Step 11: 7750 + 775 = 8525
Step 12: 8525 + 775 = 9300
Step 13: 9300 + 775 = 10075
Step 14: 10075 + 775 = 10850
Step 15: 10850 + 775 = 11625
Step 16: 11625 + 775 = 12400
Step 17: 12400 + 775 = 13175
Step 18: 13175 + 775 = 13950
Step 19: 13950 + 775 = 14725
Step 20: 14725 + 775 = 15500
Step 21: 15500 + 775 = 16275

So, 775*21 = 16275